Beamforming
The first implementations of 5G base stations will most likely use so-called analog beamforming. This is due to the higher complexity, mostly from a hardware perspective, of implementing so-called digital beamforming. The latter imposes fewer functionality restrictions but is rather costlier to realize.
Beamforming refers to a transmitter amplifying the sent signals in a selected direction while weakening it in others, and correspondingly that the receiver can amplify signals from a selected direction while weakening unwanted signals from other directions. Analog beamforming means that this can only be applied to one direction at a time by each transmitter/receiver. An array of multiple transmit antennas or receive antennas have to be used to transmit or receive in multiple directions at a time. The same signal is transmitted from multiple transmit antennas, but with individually adjusted phase shifts, which effectively creates a beam in the resulting transmit radiation pattern of the signal. The beam direction depends on the phase shifts of the antenna elements. Similarly, the phase shifts can in the receive direction be used to steer the maximal antenna sensitivity toward a desired direction.
Beamforming allows the received signal to be stronger for an individual connection, thereby enhancing throughput and coverage for that connection. It also allows reducing the interference from unwanted signals, thereby enabling several simultaneous transmissions over multiple individual connections using the same time-frequency resource, so called multi-user multiple-input-multiple-output (MIMO).
Reference Signals
One important problem with beamforming is the decision of which beam(s) (i.e., which direction) to use for transmission and/or reception. To support base station beamforming a number of reference signals can be transmitted in different beam directions, respectively, from the base station, whereby the UE can measure these reference signals and report the measurement results to the base station. The base station can then use these measurements to decide which beam(s) to use for shared data transmission to one or more UEs. A network can use a combination of persistent and dynamic reference signals for this purpose, see further details below.
The persistent reference signals, called beam reference signals (BRS), are transmitted repeatedly in a large number of different beam directions. This allows a UE to measure the BRS when transmitted in different beams, without any special arrangement for that UE from the base station perspective. The UE reports the received powers for different BRS back to the base station plus the index of the BRS, given by e.g. the BRS sequence and the time and frequency position of the particular BRS. By reporting a BRS index and an associated received power of that BRS the UE is effectively reporting its preferred beam. The UE may report a list of BRS indices and associated powers, e.g. its top eight strongest BRSs.
The base station can then transmit dedicated reference signals to that UE, using one or more beams or beam directions that were reported as strong for that UE. These are dedicated reference signals and may thus only be present when the UE has data to receive, and they give more detailed feedback information of the beamformed channel, such as co-phasing information of the polarizations and the recommended transport block size. Since the BRS is transmitted repeatedly over a large number of beams, the repetition period has to be relatively long to avoid using too much resource overhead for the BRS transmissions.
The dynamic reference signals, called channel-state information reference signals (CSI-RS), are transmitted only when needed for a particular connection. The decision when and how to transmit the CSI-RS is made by the base station and signaled to the involved UEs using a so-called measurement grant. When the UE receives a measurement grant it measures on the corresponding CSI-RS. The base station can choose to transmit CSI-RS to a UE using only beam(s) that are known to be strong for that UE, to allow the UE to report more detailed information about those beams. Alternatively, the base station can choose to transmit CSI-RS also using beam(s) that are not known to be strong for that UE, for instance to enable fast detection of new beam(s) in case the UE is moving.
The 5G base stations transmit other reference signals as well. For instance, they transmit a so-called demodulation reference signal (DMRS) when transmitting control information or data to a UE. Such transmissions are typically made using beam(s) that are known to be strong for that UE.
In 4G systems, discovery reference signals (DRS) may be used for the same purpose as BRS, as described above. Hence, the LTE UE is configured to perform received power measurement on a set of different DRS signals and report the associated DRS index and measured power for the eight DRS measurements with highest power. Hence, the proposals in this IvD are equally well applicable to 4G.
UE Beamforming
Beamforming is not restricted to the base stations. It can also be implemented in the receiver of the UE, further enhancing the received signal and suppressing interfering signals. Likewise, the UE may use transmit beamforming. Similar to a base station, analog beamforming can be used in the UE, which means that the UE can only receive/transmit from/to one direction at a time, unless multiple receivers/transmitters are available.
When operating with the 5G base stations, a UE with analog receive beamforming can measure the BRS using different UE receive beams, and then choose the UE receive beam(s) that provides the highest BRSRP (Beam Reference Signal Received Power). However, care must be taken when comparing the RSRP of different receive beams since the power depends on the utilized combination of transmit and receive beams. A given receive beam may have a high BRSRP when paired with a certain transmit beam, but have a low BRSRP in combination with other transmit beams. A different receive beam may also give an equally high BRSRP when combined with a different transmit beam, but give a low BRSRP in combination with all other transmit beams.
Since the base station may not transmit all BRS at the same time (due to limitations imposed by analog beamforming, for example) but rather cycle through all transmit beams during some time window, it is important that BRSRP-values for different receive beams that are compared stem from measuring the same transmit beam, otherwise the measurements may not be comparable.
Problems with Existing Solutions
In the case where the UE chooses the preferred receive beam(s) based on the highest received BRS power, this may then present some difficulties in a mobility scenario when the UE can measure BRSs (in this context equivalent to beams) from not only the serving transmission point (TP), such as a base station, but also additional TPs. These additional TPs could be a base station other than the base station currently serving the UE. They could also be remote antenna(s) connected to the serving base station.
Both of these situations are illustrated in FIGS. 1a and 1b. FIG. 1a shows an example diagram of a UE 56 receiving beams from two distinct TP, in this case radio base stations (RBSs) (or base stations (BS)) 24. FIG. 1b shows an example diagram of UE 56 hearing two beams from multiple antennas belonging to a single TP, RBS 24.
Now, when the BRS measurements of beams originating from a non-serving TP become stronger than those from the serving TP, the UE will then adjust its UE receive beamforming based on this other TP. This new UE beamforming will result in an improved link quality and SINR from the non-serving TP whereas the link quality/SINR from the serving TP will, due to the change of UE beam direction away from the serving TP, consequently be decreased, which may be problematic.
The same problem occurs in the UE transmit beam selection if the UE transmit beam is adjusted based on the same BRS measurements as the UE receive beam. Hence, the UE transmit beam will be directed towards the non-serving TP, which may be problematic.
In case the changing of beam directions is rather slow, as could be expected in open-area propagation environments and/or during rather low mobility speeds, this should not be a major problem since the network will get continuous feedback on the received BRS powers from the UE. Hence, the network may take actions accordingly by, e.g., scheduling a transmission towards the UE from the other TP (i.e., changing the serving TP for the UE) rather from the serving TP and/or initiating a handover procedure of the UE towards the base station controlling the new TP.
There is, however, a risk that this sudden degradation in link-budget relative to the serving TP happens faster than what the network can be made aware of due to, e.g., high mobility speeds and/or a less stable propagation environment. Hence, there is a risk that the connection to the serving TP is lost before the NW has adapted to the new conditions, which may be problematic.